1. Field of the Invention
This invention relates generally to filament wound composites wherein a low density resin is used as the matrix resin and the uncured resins used in their production.
2. Prior Art
Solid propellant rocket motor cases for missile systems, spacecraft boosters and other types of large and small high performance, lightweight pressure vessels are commonly made from fiber reinforcement and various formulations of polyepoxide resins (epoxy resins) by a filament winding process. Similarly, filament winding with both polyesters and epoxy resins has made possible production of lightweight tanks, poles, piping and the like. Historically, fiberglass has been the most common reinforcement fiber, but other fibers such as carbon filaments, boron filaments, and high modulus organic polymer filaments, most significantly aramid filaments, have become increasingly useful in these composite structures to take advantage of their differing and sometimes unique physical properties.
The resins utilized are typically epoxy formulations based on diglycidyl ether-bisphenol A (DGEBA), reactive low molecular weight epoxy diluents and curing agents such as aliphatic and aromatic amines and carboxylic acid anhydrides. Both flexibilized and rigid epoxy resins have been used as matrix resins for filament wound composite structures.
While the epoxy resins have proven useful in filament wound composites, they do have drawbacks. For example, they tend to be somewhat high in density. Densities of 1.2 to 1.3 g/cc are common. Also, they generally require relatively high proportions of resin, e.g., 35+% by weight, in the final composite. Composites based on epoxy resins are somewhat polar and hydrophilic and, thus, over a period of time, tend to absorb water from the atmosphere. These problems can be serious in applications where low weight is highly desirable, such as in aerospace applications. In addition, when epoxy resins are used with high modulus organic fibers, such as aramid fibers, there is often increased crack propagation due to weakness in the fiber's radial dimension resulting in reduced strength. This strength reduction can be corrected by precoating the fibers with substantial amounts of low viscosity lubricating or release agents. This prevents a too tight bond between the epoxy and fiber and distributes loads more evenly through the aramid fiber windings. However, while the release agent solves the problem of strength reduction it adds to the problem of weight control by increasing the weight of the composite.
Thus, a need exists for an improved resin-fiber composite which is of low density and low water pick-up, yet which provides high strength. The desired resin system should have physical and chemical properties in its uncured state, such as low viscosity and good pot life, that allow its use in production scale filament winding.